1. Field of the Invention
The present invention relates to a Y/C separator circuit for separating a luminance signal and a chroma carrier signal (simply referred to as chroma signal hereinafter) from a color composite video signal (simply referred to as video signal hereinafter).
2. Description of Background Information
An example of conventional Y/C separator circuit will be explained with reference to FIG. 1 of the accompanying drawings.
FIG. 1 shows a Y/C separator circuit in which a comb filter of the 1H delay type is used.
As shown, a video signal is supplied to a 1H delay circuit 1 for delaying the video signal for a delay time corresponding to one horizontal scanning period, and to one of two input terminals of an adder circuit 2 and a substracting circuit 3, respectively. The video signal through the 1H delay line is supplied to the other one of the two input terminals of the adder circuit 2 and the other one of the two input terminals of the subtracting circuit 3 respectively.
Since the same information signals displaced from each other by the time period of 1H include chroma (chrominance) signals whose phase angles are displaced by 180 degrees with each other, the luminance signal is obtained at an output terminal of the adder circuit 2 and the chroma signal is obtained at an output terminal of the subtracting circuit 3.
A component of the luminance signal in the chroma signal band is extracted by means of a band-pass filter 4, and in turn detected by means of a level detector circuit 5. An output signal of the level detector circuit 5 is supplied to a control input terminal of a signal suppressing circuit 6 as a control signal thereof. The luminance signal is also supplied to the signal suppressing circuit 6 after passing through an adder circuit 7.
The chroma signal is supplied to a low-pass filter 8, a level detector circuit 9 and to a chroma signal output terminal. Low frequency and medium frequency components of the luminance signal contained in the chroma signal are extracted at the low-pass filter 8, and sent back to the luminance signal through the adder circuit 7. On the other hand, the chroma signal is detected in level at the level detector circuit 9, and supplied as a control signal to another control input terminal of the signal suppressing circuit 6. The signal suppressing circuit 6 having the same construction as a signal suppressing circuit 40 which will be described later, is constructed such that a trap circuit for reducing signal components in the chroma signal band is inserted in a signal transmission path of the luminance signal when both of the control signals are supplied.
When this construction, in the presence of the chroma signal, the signal suppressing circuit 6 is activated when the chroma signal leads into the luminance signal, thereby reducing the signal components of the luminance signal in the chroma signal band. Thus, it is intended to prevent the generation of the "dot-disturbance" (generation of a fine-dot pattern on the picture) which is generally caused by the leakage of the chroma signal into the luminance signal.
On the other hand, with a video signal having a luminance signal including much of high frequency components extending near to the chroma subcarrier signal and forming a picture of less horizontal line correlation, the level detectors 5 and 9 of the above circuit still generate output signals because of the presence of luminance signal components in the chroma signal band when there is no possibility of generating the dot-disturbance. As a result, the signal suppressing circuit 6 or 9 or both is activated to reduce the high frequency components of the luminance signal, and the quality of the picture is degraded. Also, with the monochromatic (black-and white) video signal which includes only the luminance signal, if the frequency of the video signal is gradually changed from top to bottom of the picture, like a V(vertical)-sweep, the leakage of the signal also occurs because of less vertical line correlation. Therefore, if the luminance signal includes signal components in the chroma signal band, the signal suppressing circuit is activated to degrade the quality of picture.